Ultrasonic welding is an industrial process involving high-frequency ultrasonic acoustic vibrations that are locally applied to workpieces being held together under pressure to create a solid-state weld. This process has applications in the electrical/electronic, automotive, aerospace, appliance, and medical industries and is commonly used for plastics and especially for joining dissimilar materials. Ultrasonic welding of thermoplastics results in local melting of the plastic due to absorption of vibration energy. The vibrations are introduced across the joint to be welded. In metals, ultrasonic welding occurs due to high-pressure dispersion of surface oxides and local motion of the materials. Vibrations are introduced along the joint being welded.
Ultrasonic welding systems typically include the following components: (i) a press to apply pressure to the two parts to be assembled under pressure; (ii) a nest or anvil where the parts are placed for allowing high frequency vibrations to be directed to the interfaces of the parts; (iii) an ultrasonic stack that includes a converter or piezoelectric transducer for converting the electrical signal into a mechanical vibration, an optional booster for modifying the amplitude of the vibration (it is also used in standard systems to clamp the stack in the press), and a sonotrode or horn for applying the mechanical vibration to the parts to be welded; (iv) an electronic ultrasonic generator or power supply delivering a high power AC signal with a frequency matching the resonance frequency of the stack; and (v) a controller for controlling the movement of the press and the delivery of the ultrasonic energy.
A power supply provides high-frequency electrical power to the piezoelectric-based transducer, creating a high-frequency mechanical vibration at the end of the transducer. This vibration is transmitted through the booster section, which may be designed to amplify the vibration, and is then transmitted to the sonotrode, which transmits the vibrations to the workpieces. The workpieces, usually two thin sheets of metal in a simple lap joint, are firmly clamped between the sonotrode and a rigid anvil by a static force. The top workpiece is gripped against the moving sonotrode by a knurled pattern on the sonotrode surface. Likewise, the bottom workpiece is gripped against the anvil by a knurled pattern on the anvil. The ultrasonic vibrations of the sonotrode, which are parallel to the workpiece surfaces, create the relative friction-like motion between the interface of the workpieces, causing the deformation, shearing, and flattening of surface asperities. Welding system components, commonly referred to as the transmission line or “stack”, are typically housed in an enclosure case that grips the welding assembly at critical locations (most commonly the anti-node) so as to not dampen the ultrasonic vibrations, and to provide a means of applying a force to and moving the assembly to bring the sonotrode into contact with the workpieces and apply the static force.
A number of parameters can affect the welding process, such as ultrasonic frequency, vibration amplitude, static force, power, energy, time, materials, part geometry, and tooling. With regard to tooling, which includes the sonotrode, welding tip, and the anvil, these components support the parts to be welded and transmit ultrasonic energy and static force. The welding tip is usually machined as an integral part of a solid sonotrode. The sonotrode is exposed to ultrasonic vibrations and resonates in frequency as “contraction” and “expansion” x times per second, with x being the frequency. The shape of the sonotrode (round, square, with teeth, profiled, etc), depends on the quantity of vibratory energy and a physical constraint for a specific application. Sonotrodes are made of titanium, aluminum or steel. For an ultrasonic welding application, the sonotrode provides energy directly to the welding contact area with little diffraction. This is particularly helpful when vibrations propagation could damage surrounding components.
There are typically two methods of mounting any ultrasonic horn, nodal and non-nodal mounting. A node is a portion of the horn that is not moving in one or more directions. With a nodal mount the horn can be held or grasped rigidly. Non-nodal mounts require some flexible elements because the horn surface is moving (vibrating). Because of the difficulties of handling the vibrations, non-nodal mounts are typically not used in the industry. Nodal mounts typically have a flange machined at a node, or a series of set-screws positioned radially around the node.
U.S. Pat. No. 8,082,966 to Short discloses an ultrasonic welding assembly comprising a sonotrode having a single weld region and two nodal regions formed on either side of the welding region. A transducer is connected to the sonotrode with a diaphragm spring disposed between the transducer and the sonotrode. Diaphragm springs are connected to low-friction bearings that are bolted to linear guides. The sonotrode floats under high loads limiting the dampening of the acoustical vibrations. A disadvantage of this system is that the diaphragm spring is disposed between the ultrasonic transducer and the sonototrode, resulting in a significant loss of vibration energy into the mounting frame.
U.S. Pat. No. 6,613,717 to McNichols et al. discloses an ultrasonic method and apparatus including a rotatable ultrasonic horn member that is operatively joined to an isolation member. Two bearing support mounts can be employed to support the horn member in a spanning bridge configuration. The mounts may be positioned generally adjacent to a second node plane provided by a second axle member, and can fixedly hold and support a second rotatable coupler which supports a booster. A disadvantage of this system is that the booster is not a rigid booster located at a nodal mount. This results in a significant loss of vibration energy and high power requirements in order to produce a weld.
What is desired, therefore, is an ultrasonic welding tool that limits the loss of vibration energy into the frame of the tool, while providing precise tool location and maximum rigidity to the vibration welding tool. What is further desired, is an ultrasonic welding tool that requires relatively low power to produce a weld.